In positioning and assembling systems, such as for example SMD (surface mounted device) automatic placement machines, the positioning unit must be moved in the X and Y directions. For the X, Y positioning axes, a space-saving portal type of construction is preferred here, in which a positioning arm itself can be moved for example in the X direction, while a slide unit arranged in the horizontal region of the positioning arm permits the moving of the positioning unit fastened thereto in the Y direction. To achieve high accelerations for positioning arms of this type, which may also be designed as a portal, with low driving power and braking power, the moved mass must be kept as small as possible. On the other hand, however, there is the necessity of making the positioning arms resistant to bending and twisting.
Bending- and twisting-resistant positioning arms for positioning and assembling systems are usually produced as welded constructions, as a high-grade steel precision-casting construction or as a composite of high-grade steel precision-cast and welded parts. Furthermore, aluminum extruded shapes, aluminum castings and ceramic materials as well as fiber composites such as glass-fiber or carbon-fiber reinforced laminates are used.
In the production of the bending- and twisting-resistant positioning arms by precision casting, first corresponding models are prepared, these so-called xe2x80x9clostxe2x80x9d models usually consisting of waxes, thermoplastic materials, urea or mixtures thereof.
CH 686 251 discloses a method for producing lightweight, bending- and twisting-resistant portals, in particular portals in automatic placement machines, in which a model is produced from a material capable of melting, dissolving out and/or burning out and is subsequently provided with a ceramic slip by coating and with a ceramic shell by subsequent drying. After removal of the model, for example by melting out, the portal is completed by firing. Disadvantages of the use of ceramic materials are on the one hand the lack of suitable, inexpensive techniques for connecting ceramic workpieces to one another and to workpieces based on a different material, for example a metallic material, and on the other hand the high brittleness of ceramic material, which under loading can easily lead to the breaking up of a portal.
Lightweight, bending-resistant materials based on metal foams are likewise known. DE 42 06 303 discloses a method for producing metal foam bodies in which a metal powder is mixed with a blowing agent powder, the powder mixture is brought to an elevated temperature in a recipient and is extruded through a die. Thereafter, the extruded part is expanded by heating, with the blowing agent powder being decomposed, and is cooled as the finished foam body.
DE 195 01 508 discloses a component for the chassis of a motor vehicle and a method for producing such a component, the component consisting of an aluminum diecasting and having a cavity profile, in the cavity of which there is a core of aluminum foam.
However, the foam-like structure of the workpieces makes it difficult to realize releasable connections between these workpieces and other workpieces. Semifinished products incorporated into the metal foam generally do not withstand strong tensile or torsional stress, since the contact surface with respect to the semifinished product is reduced on account of the structure of the metal foam.
It is an object of the invention specify a positioning arm and a flexible method for producing positioning arms, it being intended for these positioning arms to have on the one hand the highest possible resistance to bending and twisting and on the other hand the lowest possible weight. The method is at the same time intended to be particularly suitable for the production of lightweight, bending- and twisting-resistant positioning arms for positioning and assembling systems.
According to the method and apparatus of the invention for a positioning arm for positioning an assembling system, at least one core and an outer layer enclosing the core are provided. The core is formed of a metal or surrounding foam and the outer layer is formed of a non-expanded material.
In another embodiment of the apparatus and method of the invention, a positioning arm is provided for positioning and assembling systems wherein at least one core and an outer layer enclosing the core are provided. The core is formed of a non-expanded material and the outer layer is formed of a metal or ceramic foam.
Consequently, a positioning arm of the composite material: metal or ceramic foam/metallic or ceramic material is not to be regarded as hollow, like castings of steel, aluminum or ceramic, but as solid material, and therefore has a high twisting resistance and does not spring back when subjected to accelerations.
In this case, both cores of metal or ceramic foam which are surrounded by non-expanded material and cores of non-expanded material as the core which are surrounded by an outer layer of metal or ceramic foam can be realized, which leads to great flexibility in the shaping of the positioning arms.
In one embodiment of the invention, instead of one core, a plurality of cores are jointly surrounded by the outer layer, which allows more flexible shaping of the positioning arm while retaining a standard form for cores. In addition, separating walls of solid material are produced between the cores, thereby increasing the rigidity of the positioning arm. In the case of conventional sand casting methods, such separating walls are not feasible, since the sand has to be removed again after the casting.
In a further advantageous configuration of the positioning arm further flexibility is provided in the shaping of the positioning arm by the use of multilayer structures, with metal or ceramic foams and non-expanded materials alternating.
Semifinished products are at least partly arranged in the non-expanded material. This produces a solid connection of the semifinished product in the positioning arm and consequently simple connections of the positioning arm to further workpieces can be realized. For example, in this way it is possible to provide a casting around threads, which are then used for screw connections or else tubular semifinished products as a simple form of cable bushings, which when used in automatic placement machines serve for supplying power and data to placement heads attached to the positioning arm.
The use of aluminum or aluminum alloys as the metal foam and/or non-expanded material leads to particularly positioning arms by virtue of the low specific weight of aluminum. Although the modulus of elasticity of aluminum foam materials, at approx. 5 GPa, is lower than for aluminum (69 GPa), ceramic (approx. 300 GPa) or steel (approx. 210 GPa), its low density (300-1000 kg/m3) in comparison with the other materials (aluminum: 2700 kg/M3, ceramic: approx. 4000 kg/m3, steel approx. 8000 kg/m3) has the effect of producing a high specific bending resistance, which is further improved by combination with other materials.
On account of its high rigidity, a ceramic material is also suitable in an advantageous lacuna as a non-expanded material.
Exemplary embodiments of the invention are described in more detail below and are represented in the drawing.